I'm not sure I like demons doing what they do because of some magical 'taint' I always thought they were just evil humans who don't need magic to make them evil.
We already know that they corrupt a lot (most?) of them through endless torture. They were doing that to Dean during his stint, as well as his father. And after only a year relative to Earth, he was down there doing all sorts of evil torturing himself. A few years more and the process probably would have taken hold.
This is as opposed to souls that were already evil and corrupt before going to Hell, like Crowley apparently.
So sure, I don't see a problem with it. The cleansing is just wiping away all the conditioning they went through in Hell, allowing their "true" self to realize what they've done since turning into a demon. Honestly, it's probably a far worse fate for them than simply killing them, as now they have to live with the guilt and regret of everything they've done for all eternity.